1. Field of the Invention
This invention relates generally to a fluid control circuit for positioning an earthmoving blade and more particularly to a circuit for providing dual cylinder tip of the blade, dual cylinder tilt of the blade and single cylinder tilt of the blade.
2. Description of the Prior Art
Many earthmoving vehicles have a blade mounted on the front end thereof for pushing or dozing material. The blade of such vehicles is commonly mounted such that it can be raised and lowered through the use of one or more double-acting hydraulic cylinders. Additionally, the blade of some such vehicles is mounted such that it can be tilted about a horizontal axis generally perpendicular to the blade through the use of only a single hydraulic cylinder. The blade of other such vehicles is mounted such that it can be both tilted and tipped fore and aft about a horizontal axis substantially parallel with the blade through the use of dual hydraulic cylinders one at each side of the blade. Tilting is accomplished by extending one hydraulic cylinder and retracting the other hydraulic cylinder and tipping is accomplished by extending or retracting both cylinders at the same time.
One known hydraulic system for controlling actuation of the dual hydraulic cylinders uses two control valves which are in fluid communication with a selector valve which is in turn connected to the dual hydraulic cylinders. The selector valve is a two position valve with which the operator can select one of two modes of positioning the blade, i.e. dual cylinder tilting or dual cylinder tipping. A single mechanical control mechanism is connected to both of the control valves so that they are operated in unison by the vehicle operator for directing fluid to the appropriate ends of the dual cylinders as selected by the position of the selector valve. One of the problems with that system is that using two control valves adds undue cost to the system while the mechanism connecting the two control valves to a single operator control lever adds both cost and complexity thereto. Moreover, that system has only dual cylinder tilting and dual cylinder tipping capability.
It has been found that in many cases the blade can be made more versatile if the dual cylinder arrangement is provided with controls which provide a single cylinder tilt function. In such arrangement, dual cylinder tilting of the blade is utilized when rapid changing of the blade position is desired and single cylinder tilting of the blade is utilized when it is desirable to apply maximum force to only one corner of the blade.
The heretofore known arrangements having both dual cylinder and single cylinder tilting capability of the blade employ a pair of control valves with each control valve being operatively connected to a respective one of the two tilt cylinders. One of the problems encountered therewith is a problem encountered by the vehicle operator in trying to precisely position the blade. Since the position of the blade is dependent upon coordinated modulation of two separate control valves by the operator, such precise positioning of the blade is very difficult. In at least one such known arrangement, one of the control valves is solenoid actuated and thereby makes precise positioning of the blade even more difficult since the operator only has control of the direction of fluid flow to the tilt cylinder controlled by the solenoid actuated control valve and has virtually no control over the amount of fluid flow to that cylinder.
The present invention is directed to overcoming one or more of the problems as set forth above.